Process for treating thermoplastic products



Dec.24,1 940. TH i 2,226,446

PROCESS FOR TREATING THERMOPLASTIC PRODUCTS Filed Dec. 23, 1937 3 Shgets-Sheet 1 l 1 30 l 16 a; l J

114 0617501: CEAYooN SMITH ALBERT ALL EN am tom leg;

0- I G. SMITH EI'AL v 2,226,445

PROCESS FOR TREATING THERMOPLASTIC PRODUCTS Fild Dec. 23, 1937 s SheetS -Sheet 2 6 -u 61 74: @9 50 I I I I V lflvefllbm Gm YOU/V SMITH ALBERT ALLA- v 6:, J tum amt-01 m G. SM ITH ETAL Dec. 24, 1940.

I PROCESS FOR TREATING THERMOBLASTIC PRODUCTS 3 Sheets-Sheet 5 Filed Dec. 25, 195'? I110 GRAYDON e :1 501% SM TH v ti 4% attorneys Patented Dec. 24, 1940 umrso s'r rss PATENT orr cs Grsydon Smith, Cambridge, and Albert Allen,

Sharon, Masa, sssignors, by mesne assignments, to Reed-Prentice Corporation, Worcester, Mass., a corporation of Massachusetts Application December 23,1931, Serial No. 181,256

3' Claims. (Cl. 219-47) This invention relates to the treatment of any material which requires heat in molding, as, for example, wax, Celluloid, cellulose acetate, phenolcondensate products in general, and various other a thermo-plastic materials. Such material is usu-- ally supplied to the manufacturer in granular form, such as dry powder or pellets, and in manufacturing molded articles this material is forced under high pressure through an orifice into a 10 mold cavity and is heated to a plastic condition in the passage. The material is inherently a poor conductor of heat, being therefore difll'cult to heat uniformly, and it is likely to become burnt or discolored if overheated. The problem of uni 15 formly heating amass of this material under very,

high pressure and maintaining it always within the required range of temperature presents great practical difficulties.-

One of the best processes heretofore known for 20 heating and delivering a'thermoplastic product under pressure has been to force the molding material through a heated cylinder having therein a solid core, or pineapple as it is commonly called, suspended from the internal walls of the 2.: cylinder and effective to spread the charge into annular form so that the effective heating surface is thereby increased. It is difllcult however with such apparatus to secure uniform and adequate heating the entire mass and the closest so vigilance and skillful regulation is required to obtain passable results.

We have discovered a greatly improved process of handling thermoplastic products whereby we are enabled to secure close and accurate regula- 35 tion of temperature and substantially uniform distribution of heat throughout the product by the employment of a more compact and convenient apparatus than has been heretofore available.

In one aspect our invention comprises a novel process characterized by passing granular thermoplastic material through one or more passages providing a substantial surface heating contact for the material in a ferro-magn'etic member 10- 4g outed-in the magnetic field of an alternating current system and thus adapted to be heated. by

core losses,- that is to say, by hysteresis and to some extent by induced eddy currents.

In another aspect our invention consists in so novel apparatus herein shown as adapted for carrying out the process of the invention. This apparatus may comprise an elongated body of magnetic metal of suitable cross section containing a multiplicity of longitudinal passages for the ther- 2 moplastic material, the body being located as the core of an electro-magnetic system and interposed between the mold and the pressure-generating element of the system. Such apparatus as compared with that heretofore known is much more compact and convenient in its dimensions, 5 presents a greater volume of material more eflectively distributed for heating, and provides asubstantially larger cross-sectional passage and surface heating area for the Product. l

The elongated body may take any convenient shape for incorporation in the complete system so long as it is of magnetic metal and is provided with a longitudinal passage or passages providing a considerable heating surface for the molding product. It may be located as the core of an electro-magnet or it may be shaped to carry windings so that it becomes itself an electromagnet. In any case the effect of an alternating magnetic field upon the body is .to heat it by internally developed heat and maintain it at a substantially uniform temperature which may be readily adjusted and held well within the temperature limits suited for the product passed through it. In order to avoid burning the plastic material, as commonly occurs from uneven heating, it is important to make the heating effect from hysteresis large as compared to the heating effect from eddy currents developed in the body of the conduit. Eddy currents tend to concentrate their heating veilfect on its outside surface, while the heating effect of hysteresis tends to be uniform throughout the mass of the body. Accordingly we find it very desirable to select a magnetic metal which exhibits a high hysteresis loss when subjectedto an alternating magnetic field and, to reduce eddy currents by selecting material which also has high specific resistance. Resistance to eddy currents is also effected by the geometry of the conduit body, and properly placed non-magnetic gaps in the body, such as slots, are very effective in reducing eddy currents by increasing the length of the path through which they must'fiow. In the apparatus of our invention, accordingly, the heating effect a due to eddy currents is small as compared to the heating efiect due to the hysteresis loss of the magnetic metal of the conduit body.

These and other features of the invention will be best understood and appreciated from the 1301- lowing description of several forms of apparatus illustrated in the accompanying drawings and adapted to carry out our invention, in which Fig. 1 illustrates inside elevation one embodiment of an apparatus for carrying out our inu vention, a portion. thereof being broken away and cooperating parts being shown in broken lines,

Fig. 2 is a perspective view of the apparatus, the front end portion thereof being illustrated as broken away,

Fig. 3 is a rear end elevation thereof,

Fig. 4 is a fragmentary perspective rear end view of a modified form of the invention,

Fig. 5 is a cross-sectional view through another form of apparatus,

Fig. 6 is a longitudinal sectional View of a further and preferred modified form of apparatus,

Figs. 7 and 8 are cross-sectional views taken respectively on lines 1-! and 8-8 of Fig. 6,

Fig. 9 is a cross-sectional perspective view of a further modified form of the invention,

Fig. 10 is a longitudinal sectional view of a nozzle structure employing our invention,

Fig. 11 is a longitudinal sectional view of a preferred nozzle structure employing our invention,

Fig. 12 is a sectional view taken on line i2-|2 of Fig. 11, and

Fig. 13 is a front end view of Fig. 11.

Whereas devices heretofore employed for heat ing molding material in or preparatory to molding the same have depended upon the conduction of heat from the outside, the basic feature or process of our invention resides in generating heat for the molding process within the metallic body of the device. In accordance with the invention, the heat is generated by an alternating electro-magnetic field within which the metallic conduit body is located, a passage or a plurality of independent or interconnected passages being formed through the conduit body for receiving the molding material in a manner providing a substantial surface heating areafor the contact by the material. Such construction provides a streamlined and unobstructed passage for the material and the large surface heating area serves very effectively to transfer heat to the material uniformly during its passage through the device. The device may be constructed in various forms to carry out the object of the invention, several proposed forms being illustrated in the accompanying drawings.

Referring first to the form of invention illustrated in Figs. 1-3, I5 indicates an elongated conduit body of magnetic metal provided with a plurality of longitudinal passages l8 therethrougii from end to end. Thebody may be of any suit able shape, illustrated in the drawings as hexagonal, and may have annular shoulders l8 and i? integral therewith respectively and projecting transversely at its rear and front ends, these shoulders adapting the body for connection to cooperating mechanism. The passages iii are streamlined and are illustrated as being of uniform diameter throughout the body 15 between the shoulders l6 and I! but preferably enlarged or flared outwardly at both ends thereof. Mounted on the body i5 are laminated field core pieces 20 held together by bolts 23 and having opposed inwardly directed poles 2| and 22, the ends of the poles being V-recessed and in contact with opposed corners of the hexagonal body. Field coils 24 are wound about the poles as illustrated and when the coils are energized by an alternating electric current magnetism as induced in the two closed magnetic circuits provided by the two sides of the laminated core and the body of the magnetic conduit IS.

A tubular member 25 is connected to the rear end of the body IS, a flanged sleeve 26 threaded to the member and overlapping the shoulder l6 serving to provide the connection. To the forward end of the body is connected a nozzle head i2 comprising a nozzle piece 28, a bushing 29 and a retaining sleeve 30, the sleeve being threaded to the annular flange H of the body H5. The tubular member 25 together with the body I! and nozzle head are movable longitudinally into cooperating engagement with a molding die-holding block or platen l3. A plunger operable within the member 25 in well known manner is adapted to force the molding material into and through the passages l8 of the body member. This material is ordinarily of granular form, ranging from a powder to pellets of substantial size, and a very considerable pressure is required to force the same through the heating unit.

It will be apparent that the passing of an alternating current through the coils 24 sets up an alternating electro-magnetic field in the body i5. Furthermore, the alternating electro-magnetic field thus induced causes heating of this body due to core losses, that is chiefly by hysteresis and partly by induced eddy currents, this heat being generated within the body i 5 and being substantially uniform therethrough. It is of considerable importance that the shoulder or header l6 of the body is heated since such heating substantially reduces the very great pressure required to force the solid material into and through the body, it being apparent that h eating of the material as it is jammed against the header causes a softening of the material whereby making it easier for it to enter the passages l8. The passage of the material is further facilitated by the enlarged ends of the passages 18 at the header ends thereof and by the unobstructed streamlined course of the passages. Thus, as the material is forced through the conduit body l5 its contact with a substantial surface area of the body renders the material uniformly plastic and the heating thereof can be so controlled as most efficiently to serve the p pose of the invention.

The body l5 may be made in any desirable and suitable shape or form, as will be readily understood, and in Fig. 4 we have illustrated a modified construction thereof. This body 32 comprises a central body portion and a plurality of vanes or ribs 34 radiating therefrom and providing gaps therebetween. The body portion. vanes have passages 35 extending longitudinally therethrough and these passages are illustrated as converging forwardly to compact the material as it moves through the device, it being understood that such construction may be desirable in some cases and with some materials. Heat is generated within this body by electro-magnetic forces induced by current passed through a coil or coils 36 wound about the vanes 34. I

As described above, the molding material is heated during its passage through the device by its contact with the walls of the passages and, while we have illustrated and may prefer to employ a plurality of independent passages 88 or For exammay be considered as employing a main central 75 passage 40 and a plurality of outer (as 42 in communication with the central passage. the important requirement being to provide a substantial surface contact for the material as it passes longitudinally through the device.

The preferred form of the invention which we have so far devised is illustrated in Figs. 6-8 of the drawings, it being understood however that the details thereof may be considerably modified. This device employs a conduit body 50 of magnetic metal having a large annular flange 52 at its rear end secured by tap bolts 53 to a member 54 carrying a tubular member 50, a forcing piston for the moldingmaterial operating within the member 50. A nozzle head 50 is secured by tap bolts 50 to the flange 6| on a front end of the body 50. Passages 52, similar to the passages I8, are formed through the body. On the intermediate portion of the body, between the enlarged ends of the passages is wound asbestos insulation providing a heat insulating sleeve 64. This asbestos sleeve is located between the annular flanges ,52 and SI and surrounding this sleeve between the flanges are one or more fleld coils 65. These coils may be left fully exposed or may be partly or fully housed. We have illustrated the coils as housed between sectional sleeves 68 of porcelain or other suitable insulation. u

Two laminated units 10 are mounted transversely or circumferentially on the outer sleeve 68 at opposite sides of the body 50, each un'it comprising a stack of steel plates preferably secured together by rivets or otherwise into a unit. The intermediate portion of each unit is secured to the bodyby two bolts I2 threaded into the flanged ends 52 and 6| of the body and the lateral ends of'the units are engaged beneath aluminum covers I4 secured to the body by tap bolts IS, the covers filling the oppositely disposed gaps between the laminated units 10. Each cover I4 is held to the body by four tap bolts 18 threaded into the flanges 52 and 0|. The covers are hollow andlformed with holes 'I'I permitting a free circulation of cooling air to the coils 50 and units 10, and the top cover (Fig. '7) provides a supporting outlet for the terminals I0 of the coils. The covers serve to hold the laminated units 10 in the position illustrated and prevent vibration thereof. It will be understood that these units serve to complete the magnetic circuit in which the conduit body 50 is included, that is, each laminated unit I0 contacts with the conduit body at its end flanges and bridges the intermediate space where the coils 66 are located. Thus each unit completes a closed magnetic circuit of which the conduit body is a common element and thus the magnetic flux in the conduit body is increased and the hysteresis effect intensified.

The operation of this form of the invention is generally the same as has been above described in reference to Figs. 1- 3. The passing of an alternating current through the coil 00 develops an alternating magnetic fleld within the body 50, whereupon the body is heated uniformly throughout by the resulting core losses, principally hysteresis. The body is furthermore radially slotted at 00 between the passages 52 for the purpose of breaking up and somewhat reducing the flow of eddy currents throughout the body 50, thereby obtaining a more uniform distribution of the core losses with even heatingof the body 50 and also the thermoplastic material in the passages 02.

In Fig. 9, we have illustrated a further modified form of our invention wherein the passage core losses.

through the body a is broken up by a plurality of rods 02 secured at their ends to the end walls of the body and extending longitudinally through the passage. These rods provide a substantial heating surface for contact with the material as u it is forced through the passage.

In Fig. 10 there is shown an arrangement for heating the nozzle 20 so that there will be no appreciable cooling of the plastic material as it flows through the nozzle opening into the die. For this purpose, the nozzle head 50a is composed of ferro-magnetic material and has a reduoed portion 03 surrounded by a coil 04. Upon energization of the coil 94 by alternating current, the head 50a and nozzle 20 will be heated by Since the head 50a and the nozzle 20 provide but a single central passage for the plastic materiahthis heating is suiiicient to prevent any appreciable cooling of the material before it enters the die. While we have herein illustrated the heating unit as comprising a coil without laminated units, such as the units I0 of Fig. 6, for completing the magnetic path, it should be understood that ordinarily the employment of such units is desirable since they serve to give greater heating efliciency, it being understood that both the main body and nozzle or either may be constructed with or without units for comand the nozzle is held in place by a plate I00 threaded into the forward end of the chamber and in engagement with an annular flange I I0 on the nozzle.

Surrounding the nozzle is a brass sleeve II2 slotted longitudinally at -I I4 to prevent eddy currents. The purpose of the invention is to heat the nozzle and walls of the passage I05. and to this end a coil H5 is provided within the chamber I02 and about the nozzle, the coil preferably surrounding the sleeve and nozzle as illustrated. We also preferably provide laminated steel units H5, H1 and H8 about the coil to facilitate the flow of magnetic flux through the nozzle. The unit I" may comprise a metal ribbon wound spirally about the coil and the units H0 and H5 may be metal discs covering theends of thecoil. A split wir ring I20 is arranged to snap into 'an annular groove I22 to hold the discs in place.

The coil terminals pass outwardly to supporting studs I24 within a box I25 on the head I00.

The object of the invention is to keep the walls of the passage I05 heated while the plastic composition is being forced therethrough and the arrangement of the coil and laminated units thereabout within the chamber I02 is adapted to provide such heating with greater eihciency.

It will furthermore be noted that the construction permits removal of the nozzle I04 upon removing the threaded plate I00, all without dis- 'turbing the coil and its terminals within the or alloy having marked magnetic properties and customarily used in the manufacture of electrical apparatus. The particular term-magnetic material utilized for the bodies 15, 88, 50, 58a and I is chosen for the relatively high hysteresis loss which it exhibits when subjected to an alternating magnetic field. We desire furthermore to state that, while in describing our invention herein we have more specifically referred to an alternating flux, a heating eflect will be produced by any varying magnetic flux and a pulsating unidirectional flux or random changes would be effected in producing the heating results described. It is therefore to be understood that the broad concept of the invention includes any magnetic field which will produce the required heat. While we have also herein more specifically referred to the heating of thermoplastic and granular materials, it is apparent that the invention is applicable to the heating of any form of molding material capable of being rendered plastic by heat and the term "thermoplastic is accordingly used herein in that broad sense. Furthermore, we desire it to be understood that distribution of the. core losses will not be entirely uniform and that in this respect the term unifor is intended to be construed as meaning substantially uniform. The novel apparatus herein disclosed is not claimed herein but forms the subject matter of our copending application, Ser. No. 258,494, filed February 25, 1939.

Having thus described our invention, what we claim as new and desire to secure by Letters Patent of the United States is:

1. The process of preparing thermoplastic material for molding which consists in advancing it in a narrow stream through a conduit while uniformly heating the walls of the conduit by hysteresis induced by a varying electro-megnetic field, and preventing the uneven surface heating of the material during the process by limiting the formation of eddy current at and about the walls of the conduit.

2. The process of preparing thermoplastic material for molding which consists in dividing the material into a plurality of narrow streams and thus forcing it through a plurality of conduits while uniformly heating the walls of the conduits by hysteresis induced by a varying electro-magnetic field, and preventing uneven surface heating of the material during the process by limiting the formation of eddy currents about and between the conduits.

3. The process of preparing thermoplastic or the like material for molding which consists in advancing it in a stream through a conduit with walls shaped to make extensive surface contact with the material, while uniformly heating the said walls of the conduit by hysteresis induced by a varying electro-magnetic field, and preventing uneven surface heating of the material during its passage through the conduit by limiting the formation of eddy currents at or about the walls of the conduit.

GRAYDON SMITH. ALBERT ALLEN. 

